I found a donation machine in a shop.
What does it do apart from take my money, if anything? I noticed it seems to break after a few coins.
Donation machine can unlock certain items and upgrade your shop, allowing it to sell more items.
List of unlockable stuff:
In the Afterbirth DLC, you can also find the Greed Machine, which can unlock items and achievements. It appears after Ultra Greed Fight
List of unlocks from the Greed Machine:
In addition to upgrading the shop, you can also use a bomb on it to get some coins out of the donated money pool. Especially useful if you only need a few coins to buy that shop item.
It seems that using a bomb "destroys" some coins
I noticed that the number of coins donated remains from run to run, and when it reached 10 I got the secret "The Blue Map has Appeared in the Basement!" Hopefully this helps.